1. Field of the Invention
The present invention relates to a light source and more particularly a light source for image reading apparatuses.
2. Prior Art of the Invention
Apparatuses such as a copying machine, scanner, printer, facsimile and facsimile-printer-combined multi-function printer are provided with an image reading apparatus that reads such images as characters and designs drawn on paper, etc. (hereinafter “document”).
The reduction optical system (reduction CCD system) has been known as light source for the image reading apparatus. This apparatus has an advantage that even if the document is not in contact with the document table, a sharp image can be obtained by setting the focal depth deep. However, because the light source using the reduction optical system is large in size, the contact system in which information from the document is led to sensor 108 in the same magnification and in an erect image as shown in FIG. 30 comes to be used when the size reduction of thickness reduction of the light source is needed.
In other words, LED arrays 112 are arranged over document table 106 in such a way that they are bilaterally symmetrical and the light sent to and reflected form the document table 106 is received by rod lens array 121. The LED array 112 is formed of many LED elements 125 mounted on substrate 124 in the main scanning direction as shown in FIG. 31, for example. The total length of the rod lens array 121 is the same as the total length of the LED array 112, and the rod lens array 121 is formed in such a way that a plurality of cylindrical rod lenses 122 of a specific length and specific diameter are arranged and sandwiched between substrates 124 as shown in FIG. 32, for example.
In this arrangement, the distance between document table 106 and rod lens 122 can be reduced as compared with the reduction optical system (reduction CCD system), and therefore the size of the whole image reading apparatus can be substantially reduced. Furthermore, if the focal length is reduced, a still thinner type image reading apparatus can be made.
The focal length of the rod lens array 121 (rod lens 122) can be reduced by reducing the diameter of rod lens 122. But the problem is that if the diameter of rod lens 122 is reduced, there arises much optical noise such as cross talk, flare light etc. among rod lenses 122 and an image projected on sensor 108 will be unclear. And in unexamined Japanese patent application 2000-2241656, the applicants of the present patent application propose an arrangement of rod lens array with less optical noise, which will be described later.
Furthermore, because the LED array 112 is a set of point light sources, the uniformity of illuminance can not be secured unless a certain distance is maintained between document table 106 and LED array 112. Because of this, the thickness and size reduction of the contact type apparatus using the LED array 112 has its limit. In unexamined Japanese patent application 2000-2175, the applicants of the present patent application disclose a light source in which electroluminescence is used to reduce the size and thickness.
Its arrangement is as shown in FIG. 33, for example. That is, a transparent electrode layer 103 is formed on a transparent substrate 102 made of glass, a transparent resin or the like which is long in the scanning direction. And an electroluminescence layer 101 is formed on that. Furthermore, a metallic electrode layer 104 is laminated on that. And leads 111a, 111b are led out form the transparent electrode layer 103 and the metallic electrode layer 104.
As an arrangement to realize a color light source using electroluminescence, an arrangement is known in which electroluminescence layers 101r, 101g, 101b, all equal in width on the lateral direction, are laminated as shown in FIG. 34 and emit different colors—R (red), G (green) and B (blue). In the case of a color light source, either transparent electrode layer 103 or the metallic electrode layer 104 is made a common electrode for the respective color electroluminescence layers 101r, 101g, 101b. The other is formed of three individual electrodes corresponding to the respective color electroluminescence layers 101r, 101g, 101b. Leads are led out from the common electrode and individual electrodes.
The way of forming the electroluminescence layers 101 is not limited to the vapor deposition method used for formation of general thin films, but may be printing, coating etc.
In the light source in which electroluminescence is used as optical medium as described above, the emission luminance depends on the position, especially the position in the longitudinal direction of the light source because of various factors. One of them is resistance of the electrode layer (especially transparent electrode layer 103). For this reason, the emission luminance of part of electroluminescence which is much away from connecting point P between lead 111a and transparent electrode layer 3 or lead 111b and the metallic electrode layer 4 is smaller than electroluminescence near connecting point P. If the light source is not uniform in emission luminance as shown, no uniform illuminance can be obtained on the surface of a document, and the image density read by sensor 108 depends on the position in the document.
In addition, when an image of the respective colors is to be read in the same density using a color light source, the emission luminance required in electroluminescence of each color is not the same, G>R>B, for example.
To emit the electroluminescence of each color to the emission luminance required for reading an image like that, electrical adjustment is required. For that purpose, hardware-wise or software-wise addition to the light source leads to increased costs of the light source.
Difference in thickness of electroluminescence can be thought of as another cause for dependence of the emission luminance of the light source on position. That is because the luminance is in inverse proportion to the inter-electrode distance, that is, the distance between transparent electrode layer 103 and the metallic electrode layer 104. Therefore, if an electroluminescence uniform in thickness is used as light source, the scattering of emission luminance can be solved. But it is difficult to make an electroluminescence uniform in thickness.